This amazing app can tell you where seats are free on Sydney trains -- because it's weighing the carriages in real time

It doesn’t have to be like this. Picture: Moho Film/Opus Pictures/Stillking Films

A favourite app for Sydney commuters just got a whole lot smarter.

NextThere, which launched in 2014 and has been helping travellers navigate reams of timetables with ease ever since, can now tell you which part of the platform to head for if you’re looking for a less crowded cariage.

How it does this is a classic case of a smart developer simply using the tools already available.

But first, this is what a carriage with lots of seats looks like – mostly green:

Picture: NextThere

Orange means you’ll be standing and red, forget about it.

How do they do it? Simples:

The clever folks at Sydney Trains are weighing the train. Under each axle is an airbag with an air pressure meter attached — the psi reading translates to the kilogram weight of the carriage. Each night when the train goes to bed in the yard, it’s like the equivalent of hitting the ‘tare’ button at the fruit market. Then it’s just a matter of dividing that by the average weight of each commuter (which took a bit of trial and error to figure out).